Many fish that live in coral reefs light up in blue, green, red, or orange colors. They fluoresce. Many fish species do that. Such fish do not light themselves, but take up blue light and radiate it at a lower wavelength, usually as red or green light. Blue light is the only light that penetrates into deeper water.
It is not yet exactly known what all the fluorescence is for, but there are several ideas. It appears that some fish that seem well camouflaged in daylight also wear fluorescent body decorations, such as belly marks and stains. These fish often have yellow filters for their eye lens, with which they can also detect fluorescence. In this way, they are camouflaged for predators and visible to other species.
Fluorescent signals may help find the right partner. Other fish do not seem to use their fluorescence to get noticed; but to hide themselves. Some species of red-fluorescent scorpion fish can nest between tufts of algae with the same fluorescence.
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